[olabodeIdowu]

Summary

• Add an optional WAFER_VIDEO_REVIEWER_DESCRIPTION setting for conference-specific video reviewer guidance.
• Render the configured description as sanitized Markdown above the video reviewer field.
• Add tests for custom Markdown rendering and the empty default.

Testing

• python manage.py test wafer.talks.tests — 123 tests passed.
• Python syntax checks passed.
• git diff --check passed.

Closes #694